According to conventional nuclear medicine imaging, radiopharmaceuticals are introduced into a patient's body and migrate to organs, bones or tissues of interest. The radiopharmaceuticals emit gamma photons which are received by a detector. The detector generates signals based on the received gamma photons, and those signals are processed to determine the locations from which the gamma photons were emitted (i.e., the locations of the organs, bones or tissues of interest), and the intensities of the received gamma photons, which correspond to the amount of pharmaceutical uptake and the attenuation properties of organs, bones or tissues located between the locations and the detector. These locations and intensities are processed to produce a planar image of a region of the patient's body.
A gamma photon detector (also, a gamma camera) typically consists of a scintillator and light sensors optically coupled thereto. In operation, the scintillator receives (i.e., absorbs) a gamma photon and emits a number of visible light photons in response. The light photons are in turn detected by the light sensors, which may comprise photomultiplier tubes (PMTs). The PMTs absorb the light photons and produce corresponding electrons via the photoelectric effect. Each PMT multiplies the electrons it produces, resulting in an electrical pulse from each PMT whose magnitude is proportional to the energy of the original gamma photon received by the scintillator.
Continuing the above example, the scintillator may receive a second gamma photon and emit visible light photons in response. The PMTs absorb the light photons and produce electrical pulses as described above. If these electrical pulses are produced prior to decay of the electrical pulses resulting from the originally-received gamma photon, the electrical pulses are superposed, or “pile up”, on each other in the output signals of the PMTs. These piled-up pulses do not provide useful imaging information, as it is not known what portion of the piled-up pulse is attributable to a first gamma photon and what portion is attributed to a second gamma photon. The problem may be exacerbated by additional gamma photons received in close succession, whose resultant PMT pulses may pile up on any number of previously-generated pulses. The problem is further exacerbated with increasing scintillator decay time (i.e., the amount of time over which light photons are emitted in response to a received gamma photon).
To address this problem, conventional imaging systems may discard signals in which two or more pulses are piled-up on one another, or allow the detectors to saturate, thereby producing a biased count rate-dependent response. Both approaches provide sub-optimal results.